Non-existent upstream is not fatal (#30746)

A non-existent upstream should not be fatal: it could only mean it is
not deployed yet. In the meantime, it should not block the user to
rebuild anything it needs.

A warning is still emitted, to let the user decide if this is ok or not.

lib/spack/spack/database.py

@@ -1064,9 +1064,7 @@ def _read(self):
                 self._state_is_inconsistent = False
             return
         elif self.is_upstream:
-            raise UpstreamDatabaseLockingError(
-                "No database index file is present, and upstream"
-                " databases cannot generate an index file")
+            tty.warn('upstream not found: {0}'.format(self._index_path))
 
     def _add(
             self,